Endogenous activity outside the target location in Area MT predicts perceptual sensitivity in behaving marmosets

The gain of evoked responses for targets presented at perceptual threshold are predictive of whether or not the target is perceived. It is unknown whether gain modulations are spatially specific as activity in non-target locations are not often studied in the context of sensory processing. Do non-target locations also show gain modulations in spontaneous activity that are correlated with detection sensitivity? To test this, we measured neural activity (single-unit and multi-unit activity) across area MT using chronically implanted Utah arrays in common marmosets (Callithrix jacchus) performing a visual detection task. We measured spike rates (1) for neurons whose receptive fields encompassed the target location and (2) for neurons recorded on adjacent electrodes, whose receptive fields excluded the target location. We find, as expected, that the likelihood that the target is detected increases with the target-evoked spike rate. Further, we find target detection is inversely correlated with magnitude of spontaneous activity on the adjacent electrodes. Not only is the gain modulation spatially specific, but perceptual sensitivity is improved when spontaneous activity is reduced for the surrounding population. How might spontaneous activity around the target location impact detection sensitivity? One possibility is that spontaneous activity masks the evoked response, reducing the discriminability between activity in target-present and target-absent neuronal populations. To test this we next quantified, using Signal Detection Theory, how much more detectable the presence of the target was from the surrounding spontaneous activity on hits and misses. We find that the observed shifts in spontaneous activity increase the difference in d-prime between spontaneous and evoked population activity between hit and miss trials. These results show that, in addition to depending on the spiking response evoked by the target, perceptual sensitivity also depends on the level of spontaneous, undriven activity at topographic locations beyond the target.